The pathogenesis of chronic Chagas disease could not be related to lymphocyte blastogenic responsiveness to antigens of the causative trypanosome. Some suggestive associations with absence and presence of certain HLA tissue types, however, were found in chronic Chagas' patients. Although specific cell mediated immune responses to parasite antigens in patients with leishmaniasis were demonstrated they must be differentiated from non-specific T-cell stimulation which can occur. Possible auto-immune responses to retinal antigens is being studied in patients with toxoplasmic chorioretinitis. A high degree of specific IgE response as measured by histamine release from white cells challenged with microfilarial antigens was demonstrated in patients with tropical pulmonary eosinophilia. In contrast, histamine release of white cells from filarial patients with microfilaremia was low. A similar lack of immune responsiveness by lymphocyte transformation in microfilaremic patients seems related to both serum and cellular inhibitory factors. Circulating immune complexes that bind complement were demonstrated in patients with acute schistosomiasis. These complexes decreased after treatment or as the infection became chronic. Antibody response in the IgM fraction to a specific schistosome antigen was found to be characteristic of early infections in man.